There are two problems with Lisp parentheses in my opinion:
1) Humans are not that equipped to handle that level of nesting without some other aid, this is why Lisp code is usually indented.
2) Parentheses aren't just about grouping, and this is unintuitive. For example, x is not the same as (x). This is a bit like in set theory where x is not the same as {x}, but parentheses do not look like the kind of sign that would work like that.
@bananaflag, the x versus (x) distinction is also what makes this evaluator so small: the AST uses atom versus list as the dispatch boundary, so grouping and application deliberately share syntax. An infix parser moves that complexity into precedence and associativity rules; it does not eliminate it. Indentation is still essential, but that is a tooling and display issue rather than a grammar issue.